Device for dielectric constant determination



Oct. 12, 1954 E. 1.. YOUNKER 2,691,759

DEVICE FOR DIELECTRIC CONSTANT DETERMINATION Filed March 1, 1946 FIG .I

2o' CRYSTAL DETECTOR I2 IO 24 14 FIG .2

CRYSTAL DETEC/(OR INVENTOR.

ELMER L. YOUNKER W ATTORNEY Patented Oct. 12, 1954 11mm s ares PATENTorrlce Younker, Whippany, N. J assignor', by mesne assignments; to theUnited. States of Americaas representedi by the Secretary of the.

Navy

- Applicatibn Mm'ch l 1946-, Serial No; 651,298 (s-claims... (o1.3z4 s)This invention. relatesv to: a measuring; device; and? moreparticularlyxto: a. device for dielectric; constant measurement;

Heretoforeit was. known that. the dielectric constant. of a material.could be.-.determi-ned. the. wavelength of power through. the materialfrom. a. signalv generator,. the wave length ot the: power in air fromsignalgenerator, andithe; cutoff wavelength of a waveguide. containinga. sample. of the material". for dielectrio constant: measurement wereknown.

If. it were possible to investigate the; standing. wave pattern. in theportion of the.- waveguidecontaining. the sample: ofv material, thewavelength oiithe. power: flowing through the material; could bedetermined by'measuringthe distancebetweerr successive. minima. in the:standing wave pattern... investigation of: standing wave patfiterns is.ordinarily made. by inserting. a pick-up probe into. the waveguideperpendiculantm the. electric field through a thin longitudinal slotin.the center of the waveguide wall. This: method is impractical in mostdielectric constant measurements. because is impossible-to movea pickupprobe through: a-waveguide: containing a sample of material.

. Itv is. an object of this invention to. provide apparatus formeasuring the dielectric constant of. amaterial;

It; is, a further object of thi'sinvehtion. to provide apparatus fordetermining the wavelength of power flowing through a material of solidor fluid form.

These and other objects will be more apparent upon consideration of thefollowing description together with the accompanying drawing, in which:

Fig. lis a longitudinal sectional view of an embodiment of theinvention; and,

Fig. 2 is an end view of the embodiment in Fig. 1.

In Fig. 1 is disclosed a rectangular waveguide [0. A signal generator l2operating at a frequency passable in waveguide l0 and carefully matchedthereto injects power into one end thereof. A choke plunger I4 isinserted in the other end of waveguide and is movable longitudinallytherein. A second waveguide l6 of shape similar to that of waveguide I0is loosely coupled thereto by means of an aperture I8 located in thecenter of one of the wide walls of waveguide ID. A crystal detector iscoupled to the free end of waveguide It to rectify the voltages therein.The output of crystal detector 20 is fed into microammeter 22.

A tapered sample of, the. materiaL. the value of whose dielectricconstant is tobe measured, 24 is formed with cross-sectional dimensions:equalv to; those of waveguide l0 and with length equal to: at least onefull wavelength. of the power to be transferred through it- Sample 24 isreceived; by waveguide I0 as shown in Fig-.. 1,, and securedv by anysuitable means to choke.- plung r: Ht so as to-be movable therewithChoke:

-- plunger l-.4-. also acts as a. short circuit. termination. forwaveguide I0 effective. at the adjacent end. of sample 24. Power from,signal generator I.-2 flows along. waveguide [Ba and upon being.reflected by choke plunger l4: sets up a standingv wave pattern inwaveguide. l0: containing sample 24-. The characteristics of the:standing. wave pattern at .the particular point: in. samples 2.4.: whereaperture' k8; is located are observed on. meter 22;. As sample 24 andchoke plunger: L4? are: moved along-the guide through a iuil. wave.-length. of: the power being, transferred through. sample: 24;, theindicatoron meter 22-. rises: and: falls ina substantially sinusoidalpattern. An: indication of the-wavelength of the powenbeing; transferredthrough sample 24 is obtained .by observing how far choke plunger: M5 ismovect betweensuccessive minimum. readings on meter? 22. If thedistancebetween. successive minima is; designated 11,. the dielectricv constantE of: the: 1118417311211. is; given: by the equation t.) where A is thewavelength of the power from the signal generator [2 in air and is isthe cutoff wavelength of waveguide I0.

From Fig. 2, it is evident that aperture I8 is located in the widedimension b, of waveguide I0, so that none of the current paths aredistorted and thefields within waveguide. 10 are the same as they wouldhave been if aperture 18 were not present. Waveguide I6 is excited inproportion to the magnitude of the electric field in waveguide 10. Thevery loose coupling obtained is necessary to prevent an impedance frombeing reflected into waveguide l0 and interfering with the standing wavepattern therein.

The TEo,1 mode, which is the dominant mode in a rectangular waveguide,is employed in the present invention. To permit propagation of thismode, the b dimension of waveguide 10 is larger than \/2 but, in orderto exclude higher modes, is less than A. To prevent the excitation andpropagation of TE1,0 mode, waveguide 10 is made as small as practical inthe narrow dimension a, and sample 24 should have a taper 26 in theplane of the electric field as shown in Fig. 1. The exact dimensions ofthe taper 25 are not critical for dielectric constant measurement.

What is claimed is:

l. A device for measuring the dielectric constant of a samplecomprising, a waveguide of.

dimensions suitable for receiving said sample, a choke plunger insertedin one end of said waveguide and capable of attachment to one end ofsaid sample, a source of power connected to the other end of saidwaveguide for injecting power therein and matched thereto, means formeasuring the standing wave intensity at a predetermined point in saidwaveguide produced by said source of power and said choke plunger, saidsample being movable with said choke plunger longitudinally along saidwaveguide to determine the standing wave pattern in said sample.

2. A device for measuring the dielectric constant of a samplecomprising, a waveguide of dimensions suitable to receive said sample, achoke plunger inserted in one end of said waveguide capable ofattachment to one end of said sample and movable longitudinally alongsaid waveguide with said sample, a signal generator connected to theother end of said waveguide and matched thereto for injecting powertherein, a second waveguide loosely coupled at one end to said firstwaveguide, a detector connected to .the free end of said secondwaveguide, and a meter for measuring the output of said detector.

3. A device for measuring the dielectric constant of a samplecomprising, a rectangular waveguide of dimensions suitable to receivesaid sample, a choke plunger inserted in one end of said Waveguide,capable of attachment to said sample, movable longitudinally along saidwaveguide with said sample, and providing short circuit termination insaid waveguide adjacent to said sample, a signal generator of afrequency passable in said waveguide connected to the other end thereoffor injecting power therein and matched thereto, a second waveguide ofshape similar to said first waveguide and joined to a wide wall thereof,an aperture in said wide wall loosely coupling said first waveguide tosaid second waveguide, a crystal detector connected to the free end ofsaid second waveguide for rectify- 4 ing the voltages therein, and amicroammeter for measuring the output of said crystal detector.

4. In combination, a section of wave guide, a generator coupled to oneend of said section for transmitting electromagnetic energy of a knownfrequency therethrough, a dielectric member disposed within said waveguide and having crosssectional dimensions substantially equal to thoseof said guide, a detector coupled to said guide and means for movingsaid dielectric member past said detector while maintaining a shortcircuit across said guide at the end of said member remote from saidgenerator thereby to determine the distance between adjacent voltageminima of the standing wave developed within said dielectric member bythe reflection of said electromagnetic energy at said short circuit.

5. In combination, a wave guide section, a source of electromagneticenergy coupled to one end of said section, a dielectric member disposedwithin said wave guide and having cross-sectional dimensionssubstantially equal to those of said wave guide, said dielectric memberhaving a tapered end directed towards said source of energy wherebyelectromagnetic energy propagated through said guide is coupled intosaid dielectric member with a minimum of reflection, a detectorconnected to said wave guide, means for moving said dielectric memberrelative to said detector while maintaining a short-circuited wave guidetermination at the other end of said member thereby to determine thedistance between adjacent voltage minima of the standing wave producedwithin said dielectric member as a result of the electromagnetic energyreflection at said short-circuited termination.

6. In an arrangement as defined in claim 5 wherein said means for movingsaid dielectric member comprising a choke plunger mounted in the otherend of said Wave guide section, said plunger contacting the other end ofsaid dielectric member.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,358,462 Mahren Sept. 19, 1944 2,376,785 Krasik May 22, 19452,403,289 Korman July 2, 1946 2,454,530 Tiley Nov. 23, 1948

